Electroless copper plating

ABSTRACT

CERTAIN HETEROCYCLIC AROMATIC NITROGEN COMPOUNDS, WHEN ADDED TO CONVENTIONAL ELECTROLESS COPPER PLATING BATHS IN SMALL AMOUNTS, STABILIZE THESE BATHS AGAINST SPONTANEOUS DECOMPOSITION INTO METALLIC COPPER. A PARTICULAR ADVANTAGE OF THESE STABILIZING AGENTS IS THAT THEY DO NOT ADVERSELY AFFECT THE PLATING RATE. INDEED, WITH SOME COMPOUNDS THE PLATING RATE IS SIGNIFICANTLY INCREASED BY THE ADDITION OF THE STABILIZING AGENT.

3 Jan. 2, 1973 CHANGE IN PLATING RATE (MlCRONS/HR) CHANGE IN PLATINGRATE (MlCRONS/HR) L. N. SCHOENBERG 3,708,329

ELECTROLESS COPPER PLATING Filed Sept. 10, 1971 CONCENTRATION OF HIQSAIN mg/l FIG 2 1 l 1 l J o 100 200 CONCENTRATION OF P IN mg/I UnitedStates Patent Office 33%;85329 Patented Jan. 2, 1973 3,703,329ELECTROLESS COPPER PLATING Leonard Norman Sehonenberg, Livingston, N.J.,assignor to Bell Telephone Laboratories, Incorporated, Murray Hill andBerkeley Heights, NJ.

Filed Sept. 10, 1971, Ser. No. 179,421 Int. Cl. 344d 1/092; C23c 3/02U.S. Cl. 117-47 R 12 Claims ABSTRACT OF THE DISCLOSURE Certainheterocyclic aromatic nitrogen compounds, when added to conventionalelectroless copper plating baths in small amounts, stabilize these bathsagainst spontaneous decomposition into metallic copper. A particularadvantage of these stabilizing agents is that they do not adverselyaffect the plating rate. Indeed, with some compounds the plating rate issignificantly increased by the addition of the stabilizing agent.

BACKGROUND OF THE INVENTION (1) Field of the invention The invention isconcerned with electroless copper plating baths. Interest in electrolesscopper plating has increased recently because of its use in themanufacture of printed circuits.

(2) Description of the prior art Electroless copper plating processeshave found extensive use in the manufacture of electronic circuits anddevices. A persistent problem in the use of electroless copper platingbaths is the tendency of these baths to spontaneously decompose eitheron standing or while in use. This spontaneous decomposition involvespremature initiation of the plating reaction at places other than onethe surface to be plated. After initiation, the reaction continues onthe metallic copper already liberated until all the dissolved coppersalt is reduced.

Numerous methods have been described in the literature to stabilizeelectroless copper plating baths. These methods have the undesirableeffect of decreasing the plating rate of the bath. In many commercialoperations this decrease in plating rate is a distinct disadvantagebecause of the increased time required to carry out the platingoperation. Some stabilization methods which have only a moderate effecton the plating rate often require large amounts of stabilizer whichalter the basic properties of the bath in ways which are sometimesundesirable.

Because of growing interest in electroless copper plating baths for thefabrication of printed circuits and other type electronic circuits,there is considerable need for a stable bath suitable for commercialapplications.

SUMMARY OF THE INVENTION The addition of certain heterocyclic aromaticnitrogen compounds to conventional electroless copper plating bathresults in a marked increase in the stability of these baths wtihoutadverse effects on the plating rate of the bath. The inclusion of aslittle as 1 mg. per liter markedly increases the stability of theplating bath. Also, in the case of some compounds, the plating rate issignificantly increased.

The heterocyclic aromatic nitrogen compounds have up to three rings witha hydroxy group bonded directly to one of the rings. Such compounds withmore than 3 rings are not likely to be significantly soluble in copperplating solutions. An example is Z-hydroxy pyridine. Also, iodinesubstitution on the rings, such as in the compound 2-iodo-3 pyridinol,leads to further stabilization. Plating rate is enhanced by thesubstitution of a sulfonic acid group directly on the aromatic ring.Also, sulfonic acid substitution increases the solubility of thecompound in the bath. Examples are 8-hydroXy-7-iodo-5 quinolinesulifonic acid and S-hydroxy-S quinoline sulfonic acid.

Except for the stabilizer, the composition of the plating bath isconventional. The copper is included in the bath in the form of a saltsuch as copper sulfate. A complexing agent such as disodiumethylenediaminetetracetate and a reducing agent such as formaldehyde arealso included in the solution as well as sufiicient alkaline agent tomake the pH of the solution at least 11. Other substances conventionallyused in electroless copper baths may also be included as, for example,to improve the properties of the plating or increase the wettability ofthe surface being plated.

BRIEF DESCRIPTION OF THE DRAWING DETAILED DESCRIPTION (1) Glossary ofcompounds For convenient reference, some of the compounds referred to inthe disclosure are given below. The compounds are identified by theirstructural formulas and chemical names, as well as by an abbreviatedname which is used in the disclosure.

8-hydroxy-7-iodo-5 quinoline sull'onic acid 2-iod0-3 pyridinol I H P OHQ-hydroxy pyridine 8-hydroxy-5 quinoline sultonic acid S 0 3H PSA3-pyridine sultonie acid QSA 8-quinoline sultonic acid (2) Platingmechanism An understanding of the invention is aided by a briefdescription of the plating mechanism and the mechanism by which theplating baths spontaneously decompose. The essential contents of anelectroless copper plating bath are soluble cupric salt such as coppersulfate, a complexing agent such as disodium ethylenediaminetetracetate,an alkaline metal hydroxide or other agent to adjust the pH of theaqueous solution and a reducing agent such as formaldehyde. The overallplating reaction has been established to be This is a heterogeneousreaction and proceeds only on catalytic surfaces. Metallic copper isknown to be a good cotalyst for this reaction and therefore this platingreaction is said to be autocatalytic. This is in contradistinction todisplacement plating in which the surface material reacts with theplating solution.

Essentially two types of mechanisms are responsible for initiatingspontaneous decomposition of electroless copper baths. The firstinvolves the introduction or presence in the bath of ions of catalyticmetals which are reduced by the bath to the metallic state and thencatalyzes the plating reaction. For example, Pd++ is often used tosensitize surfaces to be copper plated. Some of the Pd++ ion isinevitably introduced into the copper plating bath. This ion is thenreduced to metallic Pd which in turn catalyzes the copper platingreactions. After initiation of the reaction, sufiicient metallic copperis produced by the plating reaction to catalyze subsequent decompositionof the plating solution.

The second mechanism for spontaneous decomposition involves theformation in the solution during electroless plating of small amounts ofmetallic copper or cuprous oxide particles which catalyze the platingreaction in the solution rather than on the surface to be plated.

(3) Stabilization measurements Measurements were carried out to show thestabilizing eifects of certain heterocyclic aromatic nitrogen compoundson electroless copper plating solutions. For purposes of comparison, aparticular bath composition was used throughout these experiments. Thecomposition of the bath was 0.02 M CuSO -5H O, 0.1 M disodiumethylenediaminetetracetate dihydrate, 0.4 M formaldehyde and sufficientsodium hydroxide to obtain a pH of 12.8

Electroless copper plating solutions vary somewhat erratically in theirstability. This is due largely to the fact that initiation of thespontaneous decomposition of the bath is often due to very small amountsof impurities or dust particles which are difficult to control oreliminate. In order to obtain results which reflect the stabilizinginfluence of the added stabilizer, the decomposition of the platingsolution was deliberately initiated by adding small amounts of asolution of palladium chloride in dilute hydrochloric acid. Theprocedure was as follows. Samples containing 45 ml. of the platingsolution were placed in a 75 C. constant temperature water bath. After 5minutes, 3 drops of an initiator solution were added. This initiatorsolution was made by dissolving 1 gram of palladium chloride in mls. ofconcentrated hydrochloric acid and diluting to 1 liter. Under theseconditions the plating solution without added stabilizer decomposedinstantaneously. The results where various stabilizers were added areshown in Table I. The importance of the table is to show that certainorganic additives markedly improve 4 bath stability under rather drasticconditions. Under less drastic conditions usually encountered inordinary use, much longer decomposition times are found. The table showsthe decomposition times for the electroless copper plating bath as afunction of stabilizer concentration for various organic stabilizers.

cyclic aromatic nitrogen compound with a hydroxy group substituted tothe ring such as in the case of HP enhances the stability of the platingsolution. Unfortunately, HQ forms a precipitate under conditions of theexperiment so that its stabilizing effect cannot be measured. However,the sulfonic acid derivative of this compound, namely HQSA, is solubleand has a stabilizing effect as shown in Table I. The substitution ofiodine onto one of the aromatic rings enhances the stabilizing effect ofa hydroxy substituted heterocyclic nitrogen compound, as shown by thedecomposition times of IP shown in the table. The addition of a sulfonicacid group to the aromatic ring does not remove the stabilizing effectof the compound, as can be seen from the results for HIQSA shown inTable I. The addition of the sulfonic acid group increases thesolubility of these compounds in the copper plating bath and also has anenhancing effect on the plating rate, as will be described below. Thepresence of a sulfonic acid group alone on the ring of a heterocyclicaromatic nitrogen compound has little stabilizing effect. This is shownby the results given in Table I for QSA and PSA.

(4) Plating rate studies A particular advantage of the use of thesestabilizing compounds is that the plating rate is not significantlydecreased in the process of obtaining bath stabilization. To show thisresult, plating rate measurements were made using various concentrationsof stabilizer. Two stabilizers were investigated, namely HIQSA and IP.The measurements were carried out on 2 mil thick copper rectanglesmeasuring approximately 1" x /2" on a side. These copper rectangles wereetched for 15 seconds in 20 percent nitric acid, rinsed and activatedfor one minute in a 0.1 percent palladium chloride solution and rinsedagain. Plating was carried out in 9 0 mil solutions of the copperplating bath with varying amounts of stabilizer added. The thickness ofthe copper deposits was determined by Weight gain measurements. Theresults for HIQSA are shown in FIG. 1. The graph shows the change inplating rate (from a bath with no stabilizer added) as a function ofstabilizer concentration. Plating solution without added stabilizer hasa change in plating rate of zero. As stabilizer is added, the platingrate of the bath increases to a maximum of approximately 40-50 percentover that of the plating bath without stabilizer. On continued additionof stabilizer, the plating rate decreases and eventually becomes slowerthan the bath without stabilizer. Similar results were obtained using anelectroless copper bath with composition 0.06 M CuSO -5H O, 0.3 Mdisodium ethylenediaminetetracetate dihydrate, 0.4 M formaldehyde andsufiicient sodium hydroxide to obtain a pH of 12.8. Here the maximumplating rate was obtained with a stabilizer concentration of aboutmg./l. This maximum plating rate was about 3.1 microns per hour.

FIG. 2 shows the results of plating rate studies using IP as thestabilizer. The coordinates are the same as in FIG. 1. In the case ofIF, the stabilizing effect is much greater than with HIQSA, but theplating rate is enhanced little if at all. However, IP does providesignificant stabilization without impairing plating rate.

() Composition The invention has generally been described in terms ofthe use of certain heterocyclic aromatic nitrogen compounds asstabilizers in otherwise conventional electroless copper plating baths.The copper is included in the solution in the form of a copper salt. Thebath also includes a complexing agent for the copper ions and a reducingagent. Sufiicient alkaline agent is added to make the pH of the bath atleast 11. While such a general description of the invention isjustified, certain specific compositional ranges of particularingredients are preferred for certain purposes. These preferredcompositions are given below.

(1) Insoluble cupric salt (e.g., sulfate.

nitrate, chloride 0.002 M to (2) Complexing agent, e.g., disodium 0.15M.

ethylenediaminetetracetate dihydrate (corresponding acid or tetrasodiumsalt), Rochelle salt, citric acid or alkali metal salt of citric acid0.003 M to (3) Alkaline agent such as alkali 1M.

metal hydroxide (e.g., NaOH,

KOH) The range is sufiicient to produce a pH of at (4) Formaldehyde (orother suitable least 11.

reducing agent) 0.02 M to 2 M. (5) Stabilizing agent 1 mg. to 500mgs./liter.

In addition to the above, certain other ingredients may be added toimprove bath properties or the quality of copper plating. Theseadditives are Well known to those skilled in the art. For example, smallamounts of cyanide ion may be added to improve the properties of theplating. Other additives such as organic phosphate esters may be addedto increase the wettability of the surface being plated. Generally,these additives should not exceed 5 percent.

The compositional limits will now be discussed. Below the minimum cupricsalt concentration the plating rate is unreasonably slow; above themaximum, the salt is no longer completely soluble. The concentrationrange of the complexing agent is determined largely by the cupric ionconcentration and need only be sufiicient to complex the cupric ionpresent in the solution. The pH should be at least 11 to insurereasonable plating rates and to inhibit formation of a cuprous compound.Minimum formaldehyde concentration is determined by reasonable platingrates; maximum concentration is limited by uncontrollable spontaneousdeposition by the bath.

Minimum stabilizer concentration is determined by the desired stabilityof the bath and by the concentrations of cupric salt and reducing agent.In the case where baths are to be used only over a short period of time,i.e., one hour, a concentration of 1 mg./liter is adequate. Where longshelf life is desired, concentration as high as 500 mgs./liter isdesirable. The upper limit is determined by the fact that too high astabilizer concentration will inhibit plating completely. In practicalapplications, the stabilizer concentration will usually be between 25and 400 rug/liter. A preferred concentration is determined by the factthat for each plating composition there is a stabilizer concentrationwhich maximizes plating rate and still provides bath stabilization. Forthe bath composition used in obtaining the data in FIG. 1, thispreferred stabilizer concentration is between 180 and 300 mgs./liter.For the 0.06 M copper sulfate given above, the preferred concentrationof HIQSA is between and 200 mgs./l.

(6) Other considerations Bath compositions of this invention may be usedon all substrates under all conditions in which electroless copper bathsare considered usable. Plating may be carried out on conductive surfacessuch as copper, nickel, palladium, as well as on insulating surfaces.For insulating surfaces, sensitizing and activating procedures wellknown in the art should be followed.

Plating procedures may be carried out under a variety of temperatureconditions within the range that the bath remains liquid. Increasedtemperature generally increases the plating rate and the rate ofdecomposition.

What is claimed is:

1. An electroless plating bath in which at least 95% of the solubleconstituents consists essentially of a soluble copper salt, a complexingagent, a reducing agent, an alkaline agent sufiicient to obtain a pH ofat least 11, and an organic stabilizing agent characterized in that theorganic stabilizing agent is a heterocycic aromatic nitrogen compoundhaving up to 3 rings with at least one ring-substituted hydroxyl group.

2. The bath of claim 1 in which the ring atoms other than nitrogen atomsin the organic stabilizing agent are carbon atoms.

3. The bath of claim 1 in which the organic stabilizing agent has atleast one iodine substituent attached to at least one of the rings.

4. The bath of claim 1 in which the copper salt is contained within therange from 0.002 M to 0.15 M and is selected from a group consisting ofnitrates, sulfates, chlorides, bromides and fluorides and in which thecomplexing agent is selected from a group consisting of anethylenediaminetetracetate, the disodium salt ofethylenediaminetetracetate and tetrasodiu'm salt ofethylenediaminetetracetate, Rochelle salt, citric acid and the alkalimetal salts of citric acid.

5. The bath of claim 4 where an alkali metal hydroxide is included toadjust pH.

6. The bath of claim 1 in which the concentration of the organicstabilizing agent is between 1 mg. and 500 mgs./ liter.

7. The bath of claim 6 in which the concentration of organic stabilizingagent is between 25 and 400 mg./liter.

8. The bath of claim 7 in which the concentration of the organicstabilizing agent is between 180 and 300 mg./ liter.

9. The bath of claim 1 in which the organic stabilizing agent is2-iodo-3-pyn'dinol.

10. The bath of claim 1 in which the organic stabilizing agent is8-hydroxy-7-iodo-5 quinoline sulfonic acid.

11. A method of electroless plating of copper on an insulating surfacecomprising the steps of sensitizing and activating the surface to beplated and thereafter wetting said surface with the electroless bath ofclaim 5.

12. An electroless copper plating bath stabilized against spontaneousdecomposition by presence of an organic stabilizing agent characterizedin that the stabilizing agent is a heteorcyclic aromatic nitrogencompound containing up to 3 rings and having at least onering-substituted hydroxyl group.

No references cited.

ALFRED L. LEAVITTI, Primary Examiner I. A. BELL, Assistant Examiner US.Cl. X.R.

106-1; 117-47 A, E, 47 R

